The present invention relates to the transmission and reception of orthogonal frequency division multiplexed (OFDM) signals, and more particularly, to communications systems using OFDM to communicate information in a multiple user two-way communication system.
OFDM is a bandwidth efficient multi-carrier modulation technique where a data stream is divided into a set of lower rate digital data streams, each of which is modulated onto a separate data signal. Each data signal or sub-carrier have distinct carrier frequencies. OFDM is currently used in one-way wireless broadcasts of digital television and digital radio signals where it has demonstrated its robustness to certain types of channel impairments such as multi-path fading.
In OFDM, to avoid mutual interference between the set of sub-carriers the frequency spacing Δf between sub-carriers is chosen such that over time interval T the spectrum of each sub-carrier has a null at the other sub-carrier frequencies (orthogonal sub-carriers). For a system generating baseband samples of the OFDM signal at a sampling rate of N/T and where the number of sub-carriers is less than or equal to N, this orthogonality constraint can be efficiently realized by setting Δf=1/T so that exactly N baseband samples of the OFDM signal is generated over time T. If the number of samples N over the orthogonality interval T is a power-of-two value (i.e., N=2k where k is an integer greater than zero) the process of modulating the data streams onto the set of sub-carriers can be efficiently implemented via an Inverse Fast Fourier Transform (IFFT). To recover the set of data streams from the set of sub-carriers at the OFDM receiver a Fast Fourier Transform (FFT) can be employed.
Often, the communication channel into which the transmitter 10 transmits is divided into a number of frequency sub-channels to permit multiple users to access the system. A limitation of prior OFDM systems that propose the use of multiple sub-channels that the transmitters and receivers of these systems are configured such that once the sub-channels have been allocated the sub-channels are static in that they can not be dynamically varied in quick response to changes in demands placed on system resources, leading to inefficient use of bandwidth.
Thus, there is a need for a transmitter and receiver architecture that is adapted for a multiple user communications system that enables the use of band-width efficient transmissions. More particularly, there is a need for an adequate filtering and upconversion structure to permit an OFDM transmitter to create multiple sub-channels in a dynamic fashion where the frequency position and frequency width of the sub-channels can quickly change from one time instance to the next. Similarly, a filtering and downconversion process is required at the OFDM receiver to recover the data from the various sub-channels it receives.